Assessment of the sources of variation in changes of precipitation characteristics over the Rhine basin using a linear mixed-effects model

A linear mixed-effects (LME) model is developed to discriminate the sources of variation in the changes of several precipitation characteristics over the Rhine basin as projected by an ensemble of 191 global climate model (GCM) simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The uncertainty in climate change projections originates from natural internal variability, imperfect climate models, and the unpredictability of future greenhouse gas forcing. The LME model allows for the quantification of the contribution of these sources of uncertainty as well as the interaction between greenhouse gas forcing and climate model. In addition, dependence between climate models can be accounted for by using a two-level LME model in which the GCMs are grouped according to their atmospheric circulation model. Statistical models of varied complexity are assessed by the Akaike information criterion. More than 60% of the variance of the changes in mean summer precipitation and various quantiles of 5-day summer precipitation at the end of the twenty-first century can be explained by the climate model. Differences between climate models are also the main source of uncertainty for the changes in three drought characteristics in the summer half-year. In winter, the differences between GCMs are smaller, and natural variability explains a large proportion of the variance of the changes. Natural variability is also the main source of uncertainty for the changes in two indices of extreme precipitation. The contribution of the forcing scenario to the variance of the changes is generally less than 25%.